Question About Transformers

[Stuart Polansky]

Don't forget the 750pF cap across the "upper" OPT primary. I assume it's there to equalize the winding capacitances (or something like that).

Thanks, Grover. I did see that and was wondering about its purpose. It will be included

Stuart

[Pelliott321]

Walt might chime in here, and please do!
He likes to use 400 to 600 ohm resistors to control resonances in xformers and big caps

[SoundMods]

Walt might chime in here, and please do!
He likes to use 400 to 600 ohm resistors to control resonances in xformers and big caps

Here I am chiming in. For power amps I use high-voltage (at least 5-Kv) resistors (maybe 2 to 5 Mohms) across the primary to suppress ringing.

https://www.ebay.com/itm/401564815924?_ ... BMkK-1pcNk

With output multi-taps (16, 8, 4-ohms) I use 3 Kohm power resistors on the unused output taps.

The result? Tighter sound and improved more open mid-range not to mention the rest of the bandwidth.

[Stuart Polansky]

Worth a try. Thanks Walt and Paul.

Stuart

[dberning]

What Walt is saying makes sense. A capacitor across a parasitic transformer capacitance will change the ring frequency whereas an appropriate resistor will damp it.

[SoundMods]

What Walt is saying makes sense. A capacitor across a parasitic transformer capacitance will change the ring frequency whereas an appropriate resistor will damp it.

That's been my experience with different amplifiers whether Williamson, Ultra-linear, or SET.

[David McGown]

This discussion has been thought provoking, and has me thinking about transformer issue that I had with a pair of UTC LS-55 outputs. These transformers have a set low impedance secondary windings connected in various ways for speaker level output, but also a second set of higher impedance secondaries for line level output (50 ohms to 500 ohms) . As Grover knows, I tried these in a EL34 triode Williamson and had a lot of difficulty with getting a decent 10K square wave due to ringing, where the LS-57 outputs (same transformer except for not having the high impedance secondary) is much cleaner. So was wondering if is feasible to use the high impedance secondary (say by resistively loading this secondary using a high value resistor to minimize power loss) to help dampen the ringing. When I used it before, I did not even connect up the high impedance coils, they were just left as open coils. This obviously would require experimentation.

Sorry to change the subject, but I had been thinking about this all day.

David

[Grover Gardner]

Walt might chime in here, and please do!
He likes to use 400 to 600 ohm resistors to control resonances in xformers and big caps

Here I am chiming in. For power amps I use high-voltage (at least 5-Kv) resistors (maybe 2 to 5 Mohms) across the primary to suppress ringing.

https://www.ebay.com/itm/401564815924?_ ... BMkK-1pcNk

With output multi-taps (16, 8, 4-ohms) I use 3 Kohm power resistors on the unused output taps.

The result? Tighter sound and improved more open mid-range not to mention the rest of the bandwidth.

One resistor across the whole primary? Or one acxross each half of the primary?

[SoundMods]

With output multi-taps (16, 8, 4-ohms) I use 3 Kohm power resistors on the unused output taps.

The result? Tighter sound and improved more open mid-range not to mention the rest of the bandwidth.

[/color][/quote]

One resistor across the whole primary? Or one across each half of the primary?[/quote]

Plate to plate. It is also applicable to inter-stage transformers.

[Grover Gardner]

With output multi-taps (16, 8, 4-ohms) I use 3 Kohm power resistors on the unused output taps.

The result? Tighter sound and improved more open mid-range not to mention the rest of the bandwidth.

[/color]

One resistor across the whole primary? Or one across each half of the primary?[/quote]

Plate to plate. It is also applicable to inter-stage transformers.
[/quote]

I tried this out with the resistors you linked to across the primaries of the Heyboers, and it's a nice improvement. Cleaner, better separation of instruments and voices, smoother transients, tighter bass. The effect on the Williamsons is to make it seem like those scant 20 watts go a little further without feeling the strain. Great tip, thanks Walt!